## Kilogram Gets Controversial; Why Not Split the Difference? 520 520

gbrumfiel writes

*"As Slashdot has noted, the kilogram has a problem. The SI unit is officially defined as the weight of a 130-year-old platinum-iridium cylinder in France. But the physical object appears to be getting lighter. Scientists want to replace the cylinder with a new standard based on Planck's constant, but two experiments designed to facilitate the switch keep coming up with different results. Now one researcher is proposing a solution: just average the two diverging experiments and use that value as the official definition. Not everyone thinks that averaging the two amounts to sound research: 'Deciding to just average these two results would be perfectly proper mathematics, but it would not be science,' says Michael Hart, a physicist at the University of Manchester, UK."*
## Re:Impossible (Score:5, Informative)

Such a definition is ultimately circular. The volume of water depends on pressure, which itself has a mass component.

## Re:Well, duh. (Score:5, Informative)

I think that would be a kibigram.

## Re:Impossible (Score:4, Informative)

The physical object cannot get lighter (less massive). By definition is 1kg no matter how much mass it has.Actually... it

canget lighter. Earth's gravitational field can get weaker as matter from earth is ejected or evaporates into space.It can also get lighter as Earth's atmosphere gets heavier, making it more

buoyantin earth's atmosphere.That has nothing to do with how much mass the cylinder has, because

MASSis not a measure of weight.Mass and weight are independent. Weight is due to forces applied to mass inside a gravitational field; if the field weakens or other forces are applied to the mass inside the field, the weight will decrease or increase without any change of mass.

Earth's gravitational field and atmosphere is also not uniform, so there are places (or altitudes) you can bring the same object to, and it will be lighter or heavier, with its amount of mass being the same.

## Re:Impossible (Score:3, Informative)

Technically its mass would increase, not its weight.

Sorry to be so pedantic, but that is what this entire thread is about. =P

## Re:Impossible (Score:5, Informative)

## Re:Black Cows in Scotland. (Score:0, Informative)

Worst delivery ever of an already not-too-funny joke.

Better delivery:

A mathematican, an astrophysicist and a statistician were walking along a road in Scotland. They saw a black cow. The astrophysicist said, "I guess all the cows in Scotland are black". The statistician said, "No, all we know is that there is at least one black cow in Scotland". The mathematician said, "No, All we know is, there is at least one cow in Scotland, at least one side of which is black."

## Re:Reminds me of the deer that got away (Score:4, Informative)

## Re:Metric System (Score:5, Informative)

I'm not at all sure you are serious, but enough people seriously hold this opinion that it is worth responding.

A good system of units needs:

1) Base units which are well defined and independently reconstructible (i.e. a suitably equipped lab can calibrate their equipment purely from the definition of the units.)

2) Logically constructed compound units (e.g. units of force are derived from the units of mass, time and distance.)

3) Logically constructed convenience units (e.g. kilometres for use for distances which would be an inconveniently large number of metres.)

4) To be widely used.

The initial choice of your base units is largely arbitrary - whether it was a from a not-very-accurate measure of a king's foot size or from a not-very-accurate measure of the Earth's circumference. Item (1) can be satisfied equally well (or, in the case of mass, badly) by the metric or imperial systems. The definition of the metre has long since changed from the size of the Earth to quantities measurable in a lab (as has the definition of the foot.)

The SI system (based on metric measures) beats the imperial system hands down on items 2 and 3, and because of this now has a large advantage also on item 4.

Item 2: In Imperial you might measure (heat) energy in BTU and mechanical energy in some mixture of foot-pounds-seconds, but then you need a conversion factor to compare the two. Such conversion factors are never needed in SI.

Item 3: Imperial also messes up the convenience units by having lots of weird conversion factors (e.g. an acre is (I think) a furlong by a chain. How many square feet is that? How many ounces in a ton?*) Metric uses convenience units constructed from base units via consistently named factors of 10 or 1000.

You can't use the current problems with the kilogram as a reason to prefer imperial to metric, as imperial will be just as prone to exactly the same problems. The (UK) Imperial pound is similarly defined by the mass of a unique artifact. In the US, it is defined relative to the kilogram. Mass is the last base unit which doesn't satisfy requirement (1), and the efforts to fix this are what has triggered this entire debate.

One could go a step further, and define your fundamental units in terms of fundamental physical constants (i.e. the Plank mass, Plank time and Plank distance, charge on an electron, etc.) In such a system of units, the speed of light is 1, the formula for the energy of a photon doesn't need a constant in it etc. In practice, we can't use such a system, because we can't measure (in particular) the universal gravitational constant G with sufficient accuracy. Every time we got a better measure of G, our entire system of units would need to be updated. (I.e. with current technology, this system can't satisfy requirement (1) above.)

* And how many different sorts of ounces and tons are there? It is quite a few.